The present invention relates generally to the hot dip coating of a metal strip, such as a steel strip, with a coating metal such as zinc or aluminum, and more particularly to a hot dip coating procedure which dispenses with the need for one or more strip guide rolls submerged below the surface of a bath of molten coating metal.
Steel strip is coated with a coating metal, such as zinc or aluminum, to improve the resistance of the steel strip to corrosion or oxidation. One procedure for coating steel strip is to dip the steel strip in a bath of molten coating metal. The conventional hot dip procedure is continuous and usually requires, as a preliminary processing step, pre-treating the steel strip before the strip is coated with a coating metal. Pre-treatment improves the adherence of the coating to the steel strip, and the pre-treating step can be either (a) a preliminary heating operation in a controlled atmosphere or (b) a fluxing operation in which the strip surface is conditioned with an inorganic flux.
Whatever the pre-treatment, the conventional hot dip coating procedure employs a coating step performed in a bath of molten coating metal containing one or more submerged guide rolls for changing the direction of the steel strip or otherwise guiding the strip as it undergoes the hot dip coating step. More particularly, the steel strip normally enters the bath of molten coating metal from above and moves in a direction having a substantially downward component, then passes around one or more submerged guide rolls that change the direction of the steel strip from substantially downward to substantially upward, following which the strip is withdrawn from the bath of molten coating metal as the strip moves in the upward direction.
A number of problems arise from the employment of guide rolls submerged in the bath of molten coating metal. These problems are described in detail in application Ser. No. 08/822,782 entitled "Hot Dip Coating Method And Apparatus", and the description therein is incorporated herein by reference.
Certain attempts have been made to eliminate the employment of submerged guide rolls in a hot dip coating procedure. In these attempts, the steel strip is introduced into the molten coating metal through an elongated strip passage opening in the vessel which contains the bath; the opening is typically located below the surface of the bath, and the strip is directed through the opening and through the bath along a substantially vertical, straight-line path. Conducting a strip through the bath along a straight-line path eliminates the need for submerged guide rolls to change the direction of the strip as it passes through the bath.
The elongated, strip passage opening is typically located in the bottom of the vessel containing the bath, and expedients are employed to prevent the molten metal in the bath from escaping through the strip passage opening.
Some expedients employ mechanical seals at the opening. These mechanical seals engage the side surfaces and edges of the strip as the strip moves upwardly through the opening, causing the seal to wear or break which in turn causes leakage of molten coating metal through the opening. Other problems associated with mechanical seals include freezing and large thermal gradients in the coating metal bath, and quality problems with the strip coating including irregularities in the coating thickness on the strip.
Other expedients employ electromagnetic devices that are located adjacent the strip passage opening and which develop electromagnetic forces which urge the molten metal in the bath away from the opening. These devices may prevent the escape, from the molten metal bath, of the bulk of the molten metal in the bath (bulk containment), but they still allow some leakage or dripping of molten metal from the bath through the strip passage opening, particularly at the side edges and at the ends of the elongated vessel opening. Leakage of this type can be a major problem.